1. Field of the Invention
This application relates to electrodeless fluorescent lamps, more particularly, to minimizing microwave leakage radiation by the use of an induction coil wound in the form of a toroidal helix.
2. Description of the Prior Art
Electrodeless fluorescent lamps are known in the prior art. One class of device is described by J. M. Anderson in U.S. Pat. Nos. 3,500,118 and 3,521,120. Operation of the described devices relies on the use of ferrite induction cores to transfer power into electrodeless discharges. Ferrite materials when used in such applications are characterized by considerable inefficiency. Use of ferrite materials increases core inductance to such an extent that high frequency operation is impossible, thereby making them inherently low frequency devices. Additionally, hysteresis and eddy current losses in the core result in energy loss as well as heating of the ferrite materials. At elevated temperatures (100.degree. C.-150.degree. C.) ferrite cores change from ferromagnetic to paramagnetic resulting in a substantial reduction in their permeability. With such a permeability decrease, coil inductance drops rapidly, the induced magnetic field decreases substantially and ionization of the lamp medium cannot be sustained. In radio frequency (r.f.) electrical energy sources, relying on the coil inductance as a load, thermal runaway of the output driver occurs since a very low impedance now appears across the output driver. Also, the cost of a suitable ferrite core is of the same order as the remainder of the r.f. power source. Thus, not only does use of such a core result in system inefficiency and reduce its reliability but it significantly raises the cost of the overall device as well. An alternative approach to the use of a ferrite core is described by D. Hollister in U.S. Pat. No. 4,010,400 and is based upon the technique of placing a cylindrical induction wrapped coil in a helical form around a non-conductive, non-magnetic mandrel in close proximity to an ionizable medium. Structurally, the lamp is composed of a hollow glass envelope generally of an incandescent bulb shape and having a cylindrical cavity into which the cylindrical induction coil is placed. The interior wall of the envelope is coated with a layer of fluorescent light emitting phosphor that is capable of emitting white light within the visible spectrum upon absorption of ultraviolet radiation. The ultraviolet radiation is generated upon ionization of the medium within the envelope. Typically, the gaseous charge used in such an envelope consists of mercury vapor and an inert starting gas such as argon, helium or neon. After initiating an electrodeless discharge in the ionizable medium, the discharge is maintained by coupling the medium to an r.f. magnetic induction field having a frequency and magnitude such that free electrons are accelerated to ionizing velocity within one quarter of the period of the field frequency. The field must also be of requisite strength to sustain the ionization process, placing severe requirements upon radio frequency power delivery.
A drawback in the generation of the r.f. induction field in prior art devices such as the one just described with a cylindrical induction coil is that the lines of flux are not confined within the envelope but radiate outwardly to the surrounding area. Two significant problems arise from this electromagnetic radiation.
Firstly, since the microwave radiation is not confined to the envelope, its influence will be felt in the surrounding spaces. Definite health hazards exist and negative effects to human beings can be realized with sustained exposures to such microwave radiation. The microwave radiation emission will also interfere with radio and television reception as well as other communication transmission.
Secondly, cylindrical coil flux patterns provide maximum field density in the center of the cylinder where no ionizing gas exists. Thus, maximum flux density is not utilized for coupling with the ionizable medium and in addition, a substantial portion of the field escapes from the lamp into the space surrounding the lamp. Techniques for controlling such emissions are available. By fixing the oscillator frequency within a specific range, interference may be reduced. Typically, such control is achieved using a crystal controlled oscillator. However, the attendant cost factor would severly diminish marketability of the device. To minimize power dissipation, the radio frequency oscillator has to be overdriven. However, a severe penalty is paid in that r.f. harmonics are generated, which can interfere with communication signals. Microwave shielding, having the properties of optical transparency and electrical conductivity, can be used to coat the light bulb. But the best transparent conductive film known today is not completely transparent and would, therefore, lower the overall efficiency of the lamp. Such coatings would again provide additional cost factors.